1. Field of the Art
This invention relates to a device having a rare gas discharge lamp with a rare gas sealed in a bulb in place of mercury and, in particular, a rare gas discharge lamp device suitable as a light source for a plain paper copier or a facsimile machine.
2. Description of the Related Art
In the past, fluorescent lamps, one type of low-pressure mercury vapor lamps, have been employed as light exposure sources for a plain paper copier or a facsimile machine. This type of light source requires the following requirements: (1) The light emission portion is of an elongated type to obtain a broad illumination surface; (2) The light output is high to obtain high illumination on the illumination surface; and (3) The light output of the light emission portion is uniform along the longitudinal direction to obtain a uniform illumination level on the illumination surface. In these flourescent lamps, mercury is sealed within the bulb at a partial vapor pressure of about 5.times.10.sup.-5 torrs and a rare gas, such as argon gas, is sealed within the bulb at a partial vapor pressure of several torrs to lower the starting voltage. In these lamps, a phosphor layer coated on the inner surface of the bulb is excited by ultraviolet radiation resulting from the mercury atoms within the bulb so that it produces a light emission. The light output of the fluorescent lamp depends upon the mercury vapor pressure within the bulb. The mercury vapor pressure varies depending upon the temperature. Thus the light output of the mercury-sealed fluorescent lamp varies depending upon the ambient temperature of the fluorescent lamp.
A rare gas discharge lamp in which a rare gas such as a xenon gas is filled in place of mercury has been proposed and could be employed as a light source for plain paper copier and facsimile machine. The light output level of a rare gas discharge lamp is generally lower than that of a fluorescent lamp and is affected very little by the ambient temperature. In the rare gas discharge lamp, a glow discharge is produced within the bulb and the phosphor layer on the inner surface of the bulb is excited by the ultraviolet radiation resulting from a positive column of the glow discharge, so that it produces visible light. The light output can be increased by increasing the sealing pressure of the rare gas within the bulb. It is necessary to seal, for example, a xenon gas at a high pressure of a few tens or a few hundreds of torrs in a rare gas discharge lamp to achieve practical light output level. Sealing the rare gas within the bulb at the high pressure level results in the production of a fluctuating narrow positive column. That is, the positive column of the glow discharge, extending in the longitudinal direction of the bulb, fluctuates in the direction of the diameter of the bulb and thus becomes unstable along the axis of the bulb. As a result, the light output is not constant along the longitudinal direction of the bulb due to the varying distance between the positive column and the different phosphor particles that make up the phosphor layer on the inner surface of the bulb. The result is that a very intense light emission at locations near the positive column are produced and a weak light emission at locations remote from the positive column are produced. Prior art rare gas discharge lamps, therefore, will not obtain uniform illumination because the light output level varies from location to location along the longitudinal direction of the bulb.